We have identified three gene expression signatures in peripheral blood mononuclear cells (PBMC) from children with recent-onset polyarticular and oligoarticular juvenile idiopathic arthritis (JIA) that identify subsets of patients with apparent differences in pathobiology and clinical outcomes. These signatures include: 1) a Monocyte Signature that is predominantly expressed by older (a6 years) patients with polyarthritis, and is associated with greater disease severity and more frequent treatment with anti-TNF biologies; 2) a T Cell Signature that is also predominantly expressed by older polyarthritis patients, contains genes that implicate T cell exposure to TGFp and other pro-apoptotic influences, is associated with chronically active arthritis, and is associated with increased expression of several putative genetic markers of JIA susceptibility {IGF1R, JMJD1C and PTPN2); and 3) a B Cell Signature that is expressed by younger (<6 years) patients with either oligoarthritis or polyarthritis, and implicates similar pathology between these two JIA subtypes. Our over-arching hypothesis is that these PBMC signatures detect cells that have entered the circulation after being influenced by arthritic inflammatory milieus, and therefore reflect pathological activity at sites of arthritis. Specifically, we hypothesize that 1) the Monocyte Signature detects circulating monocytes that have participated in and/or responded to joint tissue damage; 2) the T Cell Signature detects circulating T cells that have been exposed to endogenous mechanisms in arthritic joints that are aimed at suppressing joint inflammation, such as regulatory T cell activity; and 3) the B Cell Signature detects circulating B cells with abnormal activity in the context of the developing B cell repertoires in children under age 6. We will test these hypotheses in our Specific Aims: 1) Identify differences in isolated PBMC subsets that correspond to differential expression of total PBMC gene expression signatures, 2) assess relationships of PBMC gene expression signatures to synovial fluid pathology, and 3) identify gene expression biomarker surrogates of PBMC signatures in whole blood. In the long-term, we believe that improved understanding of pathobiology associated with these PBMC signatures will lead to novel molecular tools for assessing and predicting arthritis activity that will advance clinical management of oligoarticular and polyarticular JIA.